Clock and Data Recovery Having Shared Clock Generator

ABSTRACT

This disclosure provides a clock recovery circuit for a multi-lane communication system. Local clocks are recovered from the input signals using respective local CDR circuits, and associated CDR error signals are aggregated or otherwise combined. A global recovered clock for shared use by the local CDR circuits is generated at a controllable oscillation frequency as a function of a combination of the error signals from the plurality of receivers. A voltage- or current-controlled delay line can also be used to phase adjust the global recovered clock to mitigate band-limited, lane-correlated, high frequency jitter.

This application is a continuation of U.S. Utility application Ser. No.14/371,066, which was filed on Sep. 24, 2014 under 35 USC §371 as anational stage entry of Patent Cooperation Treaty Application No.PCT/US2012/028912 (filed Mar. 13, 2012), and which is herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates generally to the field of electroniccommunications and more particularly to signaling between integratedcircuit devices.

BACKGROUND

One class of digital data communication protocols use data signals thatcarry both the data stream and a data clock on a single channel. Inthese protocols, the receiving circuit includes a clock and datarecovery CDR circuit which produces a recovered clock, based typicallyon a local reference clock that has a frequency close to that of theclock carried in the data signals. The receiving circuit uses therecovered clock to set sampling times for sampling the data signals onthe channel. Phase differences between the recovered clock and the datasignals can be detected and used as feedback in the generation of therecovered clock.

One limitation on the data rate in communication channels is jittertolerance, where jitter is variation in the relative timing of thesampling times, which can be generally characterized as phase of thelocal recovered clock, and transitions in the data signals thatcorrelate with the ideal sampling times for the data signals. In CDRbased systems, jitter can arise from a number of sources. For example,some variations in the data clock due to transmitter-side circuits cancause relatively low frequency jitter. Also, power supply noise oneither the transmitter-side or the receiver-side can cause higherfrequency jitter. The CDR sampling window, or data eye, is narrowed bypoor jitter tracking, limiting the maximum data rate than can beachieved.

An object of the technology described herein is to provide a CDRcircuit, and a method for clock recovery, achieving improved jittertolerance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements.

FIG. 1 is a simplified diagram of a system including clock recoverycircuits as described herein.

FIG. 2 is a graph of phase versus time, showing how phase tracking worksin a phase interpolator based CDR, where there is a frequency offsetbetween the data clock and receiver side clock.

FIG. 3 is a simplified diagram of a system including clock recoverycircuits as described herein, including global frequency and phasecorrection.

FIG. 4 is a diagram showing a global clock recovery circuit providingfrequency offset correction and a local clock recovery circuit.

FIG. 5 is a graph of phase versus time showing an improvement inperformance relative to FIG. 2, of a clock recovery circuit includingper-lane clock recovery, for a system like that of FIG. 4.

FIG. 6 is a diagram showing an alternative global clock recovery circuitproviding frequency offset correction and phase alignment, coupled witha local clock recovery circuit.

FIG. 7 is a frequency domain model of a circuit like that of FIG. 4.

FIG. 8 is a diagram showing a global clock recovery circuit with globalphase adjustment, and a local clock recovery circuit.

FIG. 9 is a diagram showing a phase aligning, global clock recoverycircuit and multiple local clock recovery circuits.

FIG. 10 is a frequency domain model of a circuit like that of FIG. 8.

FIG. 11 is a graph showing a jitter transfer function for a circuit likethat of FIG. 9.

FIG. 12 illustrates a method for clock recovery in a multi-lane datacommunication system between devices, with shared frequency tracking.

FIG. 13 illustrates another method for clock recovery in a multi-lanedata communication system between devices, with shared, band pass phaseadjustment.

FIG. 14 illustrates yet another method for clock recovery in amulti-lane data communication system between devices, with a combinationof shared frequency tracking and shared phase adjustment.

DETAILED DESCRIPTION

Clock recovery technology is provided in which a global recovered clockis produced for a plurality of lanes, responsive to feedback from theplurality of lanes. The global recovered clock is provided to each lane,where it is used for setting the sampling times on the respective lanes.The global recovered clock can be produced using a frequency trackingcircuit, a phase tracking circuit, or a combination of frequencytracking and phase tracking circuits. The global recovered clock canalso be produced using a voltage-or current-controlled delay line inresponse to a combination of error signals from the plurality of lanes.Both of this frequency tracking circuit and delay line can be optionallyused together, and both can optionally be made part of a feedback loop,such as a locked loop. A shared frequency tracking loop can compensatefor correlated frequency offsets in the different lanes, in response toa first combination of the error signals. A shared phase tracking loopcan compensate for correlated jitter in the different lanes, as a bandpass function of a second combination of the error signals.

In some detailed embodiments, a controllable oscillator can optionallybe used to generate a timing signal at an oscillation frequency inresponse to accumulated error. A voltage controlled oscillator can beused for this purpose. Such a circuit provides for rapid clockmultiplier unit convergence to a frequency that approximates the averageof the various lanes' error signals, in a manner that minimizes ditherjitter of local clock recovery circuits. Also in some detailedembodiments, a voltage- or current-controlled delay line can optionallybe used to modify a global recovered clock responsive to collective laneerror; that is, this delay line can be used to provide low latency bandpass jitter tracking to compensate for high-frequency, lane-correlatedjitter. These optional techniques can if desired be used together toprovide for effective, low latency jitter compensation. In furtherspecific embodiments, each of a voltage controlled oscillator and/or avoltage- or current-controlled delay line can be controlled in responseto an analog voltage, providing for low latency adjustment of the globalrecovered clock.

FIG. 1 is a simplified diagram of a first integrated circuit 10 which isconnected by a plurality of communication lines 11-14 to a secondintegrated circuit 20. In a representative embodiment, the firstintegrated circuit 10 comprises a memory controller and a secondintegrated circuit 20 comprises a memory device such as a dynamic randomaccess memory (DRAM) integrated circuit. The first integrated circuit 10in this embodiment includes transmitters 31-34 for driving respectivedata signals (“S1” to “S4”), each of which include a corresponding datastream “D1” to “D4,” combined with a corresponding data clock (e.g.TCK), on the communication lines 11-14 to the second integrated circuit20. A transmit clock generator 35 in this example produces the dataclock which is provided to the transmitters 31-34 along with the datastreams where they are combined to form data signals “S1” to “S4.” Asindicated in the drawing by the ellipses associated with thecommunication channels, the number of channels between the integratedcircuits can vary from one to many. Note that in one embodiment, eachtransmitter 31-34 uses a respective data clock, which can be derived insome manner from the transmit clock generator 35 (e.g., each transmittercan transmit according to a respective phase). Note that otherembodiments also exist, i.e., the various transmitters 31-34 can be ondifferent chips transmitting to a common second integrated circuit 20.In addition, the techniques discussed below provide benefits where therespective data clocks are derived from a common source or otherwisecorrelated.

The data signals “S1” to “S4” can be produced according to a variety ofknown encoding protocols, from basic non-return to zero NRZ orManchester encoded data to other encoding techniques applied inmultilane, gigabit SERDES channels. For example, two frequently-usedencoding techniques are 8b10b coding and 64b66b coding. These andsimilar schemes take a quantum of data (e.g., 8 bits representing 256possible data values) and map these data points to a larger code space(e.g., 10 bit codes representing 1024 possible data values), relying onvalues which emphasize a certain minimum transition density fortransmitted information; because only a small subset of the larger codespace is needed to transmit valid values, that subset can be chosen in amanner that guarantees a certain density of bit value changes relativeto previously transmitted codes, e.g., a transition no less frequentlythan every two clock periods, synchronized with a clock edge, forexample. Through recovering a local clock synchronized to transitions inthe associated data signal, it becomes possible to sample the datasignals at times close to an ideal sampling time (e.g., a midpointbetween successive rising and falling edges) and so maximize thelikelihood of correct interpretation of the individual data symbols thatmake up the transmitted data. Note that the use of per-lane orper-information-signal clock recovery provides an alternative toperiodic phase calibration between the various integrated circuits,i.e., through clock recovery, each receiver self-adjusts for phase driftand other timing changes to properly sample the incoming informationsignal.

The second integrated circuit 20 shown in FIG. 1 includes a receiver oneach lane, corresponding to respective ones of the communication lines11-14. These receivers in the illustrated example include correspondingsamplers 41-44, which receive and sample the signals on thecommunication lines 11-14 at local sampling times determined usingrecovered clocks, and produce the corresponding data streams “D1” to“D4.” A global clock recovery circuit 45 produces a global recoveredclock R-CK shared by all lanes to produce a respective, local recoveredclock to control local sampling times at the respective samplers 41-44.

In some embodiments, the plurality of receivers include detectors thatprovide local error signals correlated with timing differences betweenreceived data signals and local sampling times; these error local errorsignals can be in the form of phase error signals, such as “up/downsignals” of a local CDR for the specific lane. The global clock recoverycircuit 45 can then include a circuit to generate a combined errorsignal based on a combination of the various local error signals. Ifdesired, this circuit can be embodied as a controlled loop circuit, suchas a phase locked loop (e.g. the embodiment of FIG. 4 described below)or a delay locked loop (e.g. the embodiment of FIG. 9 described below),or both. Such a loop circuit is responsive to the combined error signaland to feedback of at least a characteristic of the global recoveredclock. In FIG. 1, these local error signals (ERRs) are represented bynumeral 48.

Note that FIG. 1 shows a reference clock input (RefCK) to the globalclock recovery circuit. In some embodiments, this reference clock signalcan be generated off chip; in other embodiments, including severalprominent examples presented below, this timing signal is generatedon-chip, using a controllable oscillator that matches an originalfrequency of oscillation as closely as possible to an average of theembedded clocks received by the second integrated circuit 20. Thecombined local error signals can be converted to an analog voltage, usedto drive this controllable oscillator. For example, the various localerror signals can be summed together and integrated using an accumulatorand a delta-sigma modulator or other smoothing circuit to reducequantization noise in an output of the accumulator. Such an embodimentis shown for example in FIG. 6 below. An output from the delta-sigmamodulator can be used to generate a control voltage for a voltagecontrolled oscillator (VCO). Optionally, the global clock recoverycircuit can also include a delay locked loop coupled to receive thistiming signal. The delay locked loop can have a voltage- orcurrent-controlled delay line to produce the global recovered clock fromthe timing signal from the controllable oscillator. If desired, thevoltage- or current-controlled delay line can be made dependent on thelocal error signals as well, e.g., based on a simple sum of these errorsignals (to provide for example band-pass based correction forlane-correlated, high frequency jitter). This is to say, someembodiments use both a voltage controlled oscillator responding to anaccumulator, for quick frequency convergence with minimal dither jitter,and a voltage or current controlled delay line to further adjust theglobal recovered clock for high frequency jitter that is correlatedbetween lanes.

In the example shown FIG. 1, the global recovered clock (R-CK) isdelivered across line 49 to local clock recovery circuits 51-54associated with corresponding samplers 41-44, which use the globalrecovered clock to produce corresponding local recovered clocks.

Each lane includes error detectors for producing the local errorsignals. In this example, the local clock recovery circuits 51-54include detectors, such as a “bang-bang” phase detector or the like,that indicate errors such as phase offsets, between a local samplingtimes and the data signals on the communication lines. In a given localclock recovery circuit, these locally detected errors are applied to acontrol loop, including for example a loop filter and a digital phaseadjustment circuit, such as a multiplexer or an interpolator responsiveto digital control signals. In this example, the reference clock for thedigital phase adjuster is the global recovered clock, with each digitalphase adjuster adjusting a local clock to provide sampling times on aper-lane basis.

The local clock recovery circuits 51-54 can be individually tuned usingcontrol registers 55-58, for example. The individual tuning provides forperformance advantages that compensate for variations in the physicalcommunication paths served by the corresponding receivers. For example,control register values can be used to enable and disable parts of thelocal clock recovery loops, such as second order elements and frequencytracking. The control register values can be used to set integral gainin the local clock recovery loops to change the speed of frequencytracking. The control register values can be used to set proportionalgain in the local clock recovery loops.

As mentioned, the reference clock (RefCK) can be produced locally on thesecond integrated circuit 20, or provided from an external source,including for example a clock transmitted from the first integratedcircuit 10, an off-chip oscillator, as a system clock, or via some othersource as suits a particular embodiment. In one embodiment, the secondintegrated circuit 20 includes an embedded oscillator of a frequencyapproximately matching the expected data clock(s) carried in the datasignals. In a second embodiment, the integrated circuit 20 includes asignaling pad that is coupled to electrically receive an off-chip clockas the reference; note that as these statements imply, the techniquesapplied herein are readily applicable to a pleisiochronous system, thatis, where the reference clock is similar to but only approximatelyrelated to a clock used at a transmitter. The techniques provided hereincan also be used in a mesochronous system, such as where anoff-receiver-chip clock is based in an oscillator also used by one ormore of the transmitters that send data signals to the second integratedcircuit (e.g., a common reference clock). As noted earlier, if anembedded oscillator is used, in some embodiments, this may beimplemented as a controllable oscillator that generates a timing signalhaving a oscillation frequency dependent on the local error signals.

In an embodiment of the technology described herein, the global clockrecovery circuit 45 includes a voltage controlled oscillator (VCO) usedfor establishing the oscillation frequency of the global recoveredclock. In an embodiment of the technology described herein, the globalclock recovery circuit 45 includes an analog loop for phase adjustment,in which the analog loop can include a voltage or current controlleddelay line used for adjustment of the phase of the global recoveredclock. In an embodiment of the technology described herein, the globalclock recovery circuit 45 includes both an analog loop, which caninclude a voltage controlled oscillator, for establishing the frequencyof the global recovered clock, and an analog loop, which can include avoltage or current controlled delay line, for phase adjustment of theglobal recovered clock.

FIG. 2 is used to provide an understanding of tradeoffs betweenfrequency tracking bandwidth and high frequency jitter compensationimplicated in the design of a CDR circuit. FIG. 2 is a graph of phaseversus time, showing a first trace 90 which represents phase of atypical source clock used in a CDR (labeled as a clock multiplier unitoutput or CMU), a second trace 91 which shows the phase a localrecovered clock, e.g., the dithering output of a phase interpolator (orother phase adjuster) based on use of the source clock as a reference. Athird trace 92 shows the phase of a data clock which is embedded in adata signal of interest, and which the phase interpolator is attemptingto replicate. The phase of the local reference clock on trace 91 isadjusted by the phase increments Δ_(φ), at an update rate or frequencyf_(dig); this value defines the interval of time 1/f_(dig) between phaseadjustments. The phase increment Δ_(φ) and the frequency f_(dig) of theloop have an effect on the jitter transfer function and the trackingrange of the CDR receiver, as they translate to shifts in the samplingtimes at the respective receivers. That is to say, if the step sizeΔ_(φ) is too large, each lane will experience excessive dither jitter(which can affect correct data sample interpretation and create powersupply noise and other issues) and if the update frequency f_(dig) istoo slow, the effect can be the same (or can imply that the datafrequency is out of range of the CDR circuit). This is furtherelaborated on as follows; loop latency in conventional CDRimplementations can be 10-20 unit intervals (UI)—with such latency,increasing the proportional gain of the local loop to achieve highertracking bandwidth results in significantly higher cycle jitter as eachphase error signal produces a relatively greater change in recoveredclock phase. As a result, in typical cases, the CDR tracking bandwidthis kept less than 10 MHz as a matter of design preference. Thislimitation on tracking bandwidth reduces the receiver's tolerance tohigh frequency jitter from other sources.

Another issue for clock recovery circuits using a shared phase lockedloop PLL with per-lane digital clock and data recovery CDR circuits isthe frequency offset tracking. Assuming there is a frequency differencebetween the clock multiplier unit output (e.g., the output of a PLL,trace 90) and incoming data signals (e.g. as represented by trace 92),the CDR needs to compensate for this frequency offset. Unfortunately,larger phase step sizes are required in the local digital clock recoverycircuits to compensate for a reasonable amount of frequency offset (e.g.offsets >300 ppm). The reliance on larger phase step sizes for frequencytracking also results in higher cycle dithering jitter, degrading highfrequency jitter tolerance.

FIG. 3 illustrates features of embodiments of the technology describedherein, in which the global clock recovery circuit includes both afrequency tracker 65 and a phase adjuster 66, while the per-lane, localclock recovery circuits include local phase adjusters 71-74. Thefrequency tracker can have a voltage controlled oscillator responsive toan accumulator, to change oscillation frequency, and the global recoverycircuit phase adjuster can be implemented using a voltage or currentcontrolled delay line, responsive to a simple sum of local errorsignals. The local clock recovery circuits can be any type ofconventional clock recovery circuit, including for example, one thatrelies on a phase interpolator driven by feedback from a comparisoncircuit that compares interpolator output with transitions in the datasignal. In FIG. 3, a first integrated circuit 10 (like that of FIG. 1)is connected by a plurality of communication lines 11-14 to a secondintegrated circuit 60. The first integrated circuit 10 in thisembodiment includes transmitters 31-34 for driving respective datasignals (“S1” to “S4” are shown), each of which include a correspondingdata stream “D1” to “D4,” combined with a corresponding data clock (e.g.TCK), on the communication lines 11-14 to the second integrated circuit60. A transmit clock generator 35 in this example produces the dataclock which is provided to the transmitters 31-34 along with the datastreams where they are combined to form data signals “S1” to “S4.”

The second integrated circuit 60 includes a receiver on each one of thecommunication lines 11-14. These receivers in the illustrated exampleinclude corresponding samplers 41-44, using local recovered clocks, andproducing the corresponding data streams “D1” to “D4.” The localrecovered clocks are produced by local phase adjusters 71-74, such asdigital phase interpolators which are each arranged in a control loopresponsive to a respective local error signals associated with one ofsamplers 61-64. Each local phase adjuster 71-74 receives the globalrecovered clock across line 69. The global clock recovery circuitincludes a combination of a frequency tracker 65 and a phase adjuster66. The local error signals produced in each of the lanes correspondingwith communication lines 11-14 are delivered on line 68 to the globalclock recovery circuit, where they are combined and used to control boththe frequency tracker 65 and the phase adjuster 66. For frequencytracking, the combined error signals can be accumulated or integrated,and modulated (using e.g. Delta-Sigma Modulation) to smooth quantizationerror, for use in control of a voltage controlled oscillator trackingthe data clock. For phase tracking, the combined error signals can besummed, converted to analog and filtered to provide a control signal fora voltage or current controlled delay line.

FIG. 4 is a diagram showing a global clock recovery circuit (upperportion 98) and a local clock recovery circuit (lower portion 99), whichis replicated for each lane. The global clock recovery circuit 98provides the global recovered clock R-CK to each of the local clockrecovery circuits. Each local clock recovery circuit 99 provides errorsignals (e.g. signals on line 105) to a combining circuit or logic 110which combines the error signals from a plurality of lanes (e.g. signalson line 106) to provide a combined error signal (ERRs) to the globalclock recovery circuit 98.

In FIG. 4, the illustrated local clock recovery circuit receives theinput data signal on line 100. This signal is then proceed by an errordetector 101, such as a bang-bang phase detector. The error detector 101generates a digital error signal, which is applied to digital loopfilter 102. The digital loop filter 102 can be implemented to execute awide variety of filter functions, including first-order filter functionsor second-order filter functions. The output of the digital loop filter102 is provided as a control input to an active element 103 in the loop,such as a digital phase interpolator, which produces the local recoveredclock used for sampling the data. The output of the active element 103is fed back on line 104 to the error detector 101.

The output of the error detector 101 is applied on line 105 to the asumming node 110 that combines this output with error signals on line106 from other local clock recovery circuits. The combined error signal(ERRs) is provided to an accumulator 111, which integrates the errorsignals to produce a digital combined error signal. The digital combinederror signal at the output of the accumulator 111 is applied to theDelta-Sigma (Δ-Σ) modulator 112 (also sometimes called a Sigma-Deltamodulator). The Delta-Sigma modulator 112 produces a digital outputwhich (once the global recovered clock is locked in a manner that tracksa fundamental frequency derived from the combined, accumulated errorsignals) is dithered at a very high rate between two, or a small numberof, output levels, such that the average output value matches the input,smoothing the quantization noise of the digital input from theaccumulator 111. The output of the modulator 112 is applied to a loopdivider 126, implemented in this example by a fractional-N divider,which is part of a phase locked loop. The phase locked loop receives areference clock having a reference frequency f_(REF) at the input of aphase and frequency detector 120. The output of the detector 120 isapplied to a charge pump including a pull-up stage 121 and a pull-downstage 122, which is coupled in turn to an analog loop filter in thisexample represented by the resistor 123 and the capacitor 124. Theresulting signal is applied as a control input to a voltage controlledoscillator 125, causing the voltage controlled oscillator to trackaverage data clock frequency. The signal produced at the voltagecontrolled oscillator 125 is provided to the loop divider 126. Also, theoutput of the voltage controlled oscillator 125 is applied through clockbuffers 128 as the global recovered clock R-CK to the plurality of localclock recovery circuits, including local clock recovery circuit 99.

One can understand the benefits of improved frequency tracking for aglobal recovered clock by comparing FIG. 5 with FIG. 2. FIG. 5 is agraph of phase versus time, showing the reference clock trace 90 fromFIG. 2, and a second trace 92 which once again shows the phase of theclock embedded in the data signals of interest. Note that a third trace150 is also illustrated, representing a global reference clock from asystem like that of FIG. 4. That is to say, through use of the circuitryillustrated in that Figure, oscillation frequency itself can begenerated much closer to the desired goal (represented by the secondtrace 92), as represented by arrow 152. A fourth trace 151 representsoperation upon the global reference clock by one of the individual clockrecovery circuits. In this example, the local recovered clock has afrequency that is controlled by the global recovered clock, and itsphase is adjusted by the phase increments Δ_(φ) using per-lane, digitalphase interpolators. As illustrated by comparison with FIG. 2, the sizeof the phase increment Δ_(φ) can be much smaller because the frequencyoffset between the local recovered clocks and the data signals is keptat a very small value by an active global clock recovery circuit.

Using a global recovered clock which tracks the frequency of theincoming data signals, a substantial reduction in the phase incrementsapplied by the local clock recovery circuits is achieved. Smaller phaseincrements applied at local clock recovery circuits can further improvethe jitter transfer function at higher frequencies. The combination offrequency tracking and phase adjustment at the global clock recoverycircuit can facilitate a system having a further improved jittertransfer function.

FIG. 6 is a diagram showing a global clock recovery circuit 130 and alocal clock recovery circuit 99, where the local clock recovery circuit99 is represented by a schematic like that of FIG. 4. The referencenumerals for components shown in FIG. 4 are the same in this diagram,and are not described again. In this embodiment, the global recoveredclock (R-CK) is produced using a voltage controlled oscillator 132 in aconfiguration that is responsive to the error signals, but not tofeedback of the global recovered clock. In this example, the controlvoltage for the voltage controlled oscillator 132 is provided at theoutput of a multiplexer 131. The inputs to the multiplexer 131 includean initial calibration value or other starting frequency setting, andthe output of the Delta-Sigma modulator 112. The output of the voltagecontrolled oscillator 132 is provided through a buffer network 133 asthe global recovered clock (R-CK) to the local clock recovery circuits,such as the circuit 99. Although not illustrated, a digital-to-analogconverter and/or other filter, can be included in the circuit to smooththe variations in the control signals for the voltage controlledoscillator 132.

The circuits 98 and 130 shown in FIGS. 4 and 6, respectively, and othercircuits implementing a transfer function like that of FIG. 7,correspond to means, coupled to the plurality of receivers, for trackingfrequency of the received data signals. Such circuits can controlfrequency offsets between the global recovered clock and the datasignals. Also, the circuits shown in FIG. 8, corresponding to thefrequency tracker 301, correspond to a means, coupled to the pluralityof receivers, for tracking frequency of the received data signals. Thefunction of such means can be responsive to combinations of the localerror signals as described herein.

FIG. 7 shows a frequency domain model of a circuit like that of FIG. 4,which can be implemented using a variety of circuits. Note once againthat the bottom portion of FIG. 7 shows but one local clock recoverycircuit for ease of description, but in fact, there are typically pluralsuch circuits, one for each lane of data. Each local clock recoverycircuit receives the data phase signal φ_(DATA) on line 200 as an inputto a summing node 201. An output of the summing node 201 representsdetected phase error, and is provided to a gain circuit 202 toeffectively multiply this phase error by K_(PD). The signal is thenapplied to a loop filter, which in this embodiment includes two pathsrepresenting second order tracking. A first path (K_(P)) responds todetected phase difference, while a second path provides a second orderor integrating element 204 (K_(I)(1−z⁻¹)). The second order patheffectively tracks inability of the first path to converge on theembedded clock, and provides additional frequency adjustment (e.g.,through the use of a second phase step size or other means). Note thatin some embodiments, only one of these paths need be included or used,i.e., the embodiment of FIG. 7 can be practiced using first ordertracking only if desired. The outputs of the elements 203 and 204 areapplied to another summing node 205, and fed to a digital phasecontroller 206 (having a transfer function of K_(DPC)/(1−z⁻¹)). Theoutput of this digital phase controller 206 is then fed back via on line208 to the summing node 201 to derive detected phase error. The outputof summing node 207 identifies the phase of the local recovered clockφ_(CDR).

The global recovered clock is produced in the global clock recoverycircuit which comprises a phase locked loop responsive to theaccumulated error signals. Thus, in the diagram of FIG. 7, the errorsignal at the output of the error detector 202 is applied to anaccumulator 209A (along with error signals from other local clockrecovery circuits). The transfer function of the accumulator isrepresented as (K_(FREQ)/(1−z⁻¹)). The accumulator converts the phaseerror signals to the frequency domain, and then provides this frequencydomain signal to a second accumulator 209B, implemented as a Delta-Sigmamodulator as described above. The output of accumulator 209B provides anerror signal φ_(err). Note that the effect of the accumulators is toaverage error signals from multiple local clock recovery circuits and tointegrate those signals, e.g., the global clock is adjusted effectivelyto track average frequencies of the clocks embedded in the data signalscarried by the multiple receiver lanes.

The accumulated phase error signal φ_(err) is then applied to summingnode 226 in the phase locked loop. The output of the summing node 226 isapplied to node 221 to take the difference between the phase of thereference frequency φ_(REF) on line 220 and the output of the summingnode 226. The adjusted signal from this node 221 is applied to chargepump 222 (represented by the function I_(CP)/2π), to essentially performa digital to analog conversion. The analog output of the charge pump isthen filtered according analog loop filter 223 (R+1/Cs), and in turn,used as the control signal for a voltage controlled oscillator 224(represented by function K_(VCO)/s). The voltage controlled oscillatoroutput φ_(R-CK) is then provided to a the 1/N dividing node 225, whichin turn provides a second input to the summing node 226. Reflecting onthe operation of the circuit of FIG. 7, the voltage controlledoscillator 224 produces a signal having an oscillation frequency that isa function of the combined error signals from the various local clockrecovery circuits.

FIG. 8 is a diagram showing a global clock recovery circuit having botha frequency tracker 301 and a phase adjuster 300. The global clockrecovery circuit is used with a plurality of local clock recoverycircuits on the device (e.g., each for a different lane of data havingan embedded clock). Only one such local clock recovery circuitillustrated in this Figure. The local clock recovery circuit 99 isillustrated in the same manner as that of FIG. 4, with like referencenumerals, and not described again. The global clock recovery circuitincludes a summing node 310 that receives the error signals on line 105,and, as represented by line 309, error signals from other local clockrecovery circuits on the device. The error signals on line 105 (and 309)can be in the form of digital up/down signals such as produced using abang-bang phase detector in each local clock recovery circuit 99. Theoutput of the summing node 310, which represents a sum or instantaneousaverage of the error signals, is applied to a digital accumulator 350;the digital accumulator integrates the combination of error signals andin turn provides its output to a Delta-Sigma modulator 351. TheDelta-Sigma modulator 351 controls a fractional N divider 352 used forthe frequency tracker 301. The output of the fractional N divider 352then is applied to a phase and frequency detector 353, the other inputof which is a reference clock on line 355. An output of the phase andfrequency detector 353 is applied to charge pump, represented by thepull-up stage 356 and the pull-down stage 357, to provide an amplifiedsignal. This signal is then filtered through an RC path (i.e., resistor358 and the capacitor 359), to generate an analog control signal for avoltage controlled oscillator 360. Thus, the voltage controlledoscillator is controlled to generate a frequency of oscillation toclosely track the average frequency of embedded clock represented by thevarious data signals. By generating a timing signal reference thatclosely matches the typical embedded clock frequency, the size of thephase increments needed in the individual local clock recovery circuitsat each update time can be reduced, thereby permitting for substantialreduction in dither jitter.

The phase adjuster 300 receives the frequency tracking clock from thevoltage controlled oscillator 360 as an input to a delay locked loop(DLL). This DLL is rooted in a voltage-controlled delay line 313.Alternatively, a current-controlled delay line can instead be utilized.The output of the delay line is the global recovered clock f_(R-CK),which is applied to the local clock recovery circuits. The effect ofthis delay line is to provide a low latency, band-limited feedback pathfor further jitter reduction, i.e., a control signal from node 312 isused to adjust the global recovered clock for jitter associated with aspecific frequency band. Note that the output of the voltage controlleddelay line 313 is fed back on line 314 to a phase and frequency detector315 for the delay locked loop. The phase and frequency detector 315generates an error signal which once again is applied to a charge pumpand loop filter circuit 316 (CP+LF), and used for purposes of errorcomparison at node 312. Note that once again, the summing node (312) isused to produce an analog control voltage, although this time applied tothe voltage-controlled delay line 313 (or current for acurrent-controlled delay line). The feedback loop formed by phase andfrequency detector 315 and charge pump and loop filter 316 can act tokeep the phase of f_(R-CK) phase aligned with the frequency trackingclock from voltage controller oscillator 360 by setting the delay of thedelay line 313 within its range to be nominally be one clock period. Asecond input to summing node 312 represents the combined error signalsfrom the plurality of local clock recovery circuits. In this example,the error signals from the local clock recovery circuits are summed andprovided at the output of the summing node 310 as input to the delaylocked loop through a low pass filter 311. The low pass filter has acut-off frequency that is relatively high, compared to that of the loopfilter 102 in the local clock recovery circuits 99 and to the delaylocked loop bandwidth discussed above. The effect of this circuit istherefore to urge the delay path to center at an integer number of clockperiods, while the error signal path provides band limited corrections.Below the frequency represented by delay locked loop update frequency,the phase of f_(R-CK) depends primarily on the phase of the frequencytracking clock produced by voltage controlled oscillator 360. Thesecharacteristics contribute to establishing a band pass transfer functionfor the phase adjustment circuit. Above the delay locked loop updatefrequency and below the cutoff frequency of filter 311 (i.e. within thepass band), error signals 105 and 309 are able to add or subtract phaseto f_(R-CK) to adjust the phase of the global recovered clock.

The shared DLL can track correlated jitter from the all data lanes. Mostof the correlated jitter can be around mid-frequency (e.g. 50 MHz to 300MHz for some high speed data channels), hence a band-pass phase transferin the mid frequency range for the particular system (e.g. a 50 MHz to300 MHz pass band) would be advantageous for systems susceptible to thistype of jitter.

FIG. 9 illustrates another embodiment, where the global clock recoverycircuit comprises an analog phase adjustment circuit, withoutnecessarily including a frequency tracker like that of FIG. 8. In thisembodiment, there are a plurality of local clock recovery circuits 1, 2,. . . N, represented by block 400. A global recovered clock is producedby clock multiplier unit (CMU) 420. The CMU has a frequency selected tomatch that of the data clock carried by the data signals (e.g., it doesnot necessarily include a voltage controlled oscillator or othercontrollable oscillator, and can use any form of reference timingincluding without limitation one provided from off-chip). As with theembodiments presented earlier, local clock recovery circuits 400 producerespective error signals which are provided to on lines 401-1, . . .401-N to summing node 402, which in turn is applied as input to filter403. The summing node can if desired include voltage-to-currenttransducers for each incoming error signal line and a current summingcircuit to generate an analog output. Alternatively, a digital to analogconverter can used after digital summing with some latency penalty. Theoutput of the filter 403 is applied to a second summing node 404, andused to generate a control signal for a voltage (or current) controlleddelay line 405 (or other analog delay adjustment circuit). In variousembodiments of the analog phase adjustment circuit, the delay line 405can be less than 2.0 UI to limit its response to low frequency jitterrepresented by the combination of error signals; in some embodiments,the delay line has a range of about 1.0 to 1.5 UI. A DLL is implementedby feeding back the output of the voltage controlled delay line 405 vialine 406 to a phase and frequency detector 407. The second input of thephase and frequency detector is a reference clock provided at the outputof the CMU 420 in this example. The output of the phase and frequencydetector 407 is applied to a charge pump and loop filter circuit 408,and then to the summing node 404, to provide feedback used to controlthe delay line 405. In this example, the global recovered clock has aphase that is adjusted in response to a combination such as a simple sumor average (e.g., instantaneous sum) of the local error signals. As aresult of applying the signal representing the combination of errorsignals in analog form to adjust the control signal (at node 404), thephase adjustment circuit reacts to the jitter represented by thecombination of error signals with low latency; as a result, relativelyhigh frequency, correlated jitter from the all data lanes can becompensated. This results in a phase tracking circuit with a band-passphase transfer function, that is, where low frequency cut off is basedon the limited range of the delay line and high frequency cut off isbased on the low latency of the feedback the circuit. This circuitoperates in combination with local clock recovery circuits, thusproviding a combination approach to jitter mitigation.

The circuit illustrated in FIG. 9, other than the local clock recoverycircuits 400, corresponds to a means, coupled to the plurality ofreceivers, for aligning phase of the global recovered clock with thedata signal. Also, the circuits shown in FIG. 8, corresponding to thephase adjuster 300, correspond to means, coupled to the plurality ofreceivers, for adjusting phase of the global recovered clock in responseto a combination of the local error signals.

FIG. 10 shows a frequency domain model of a circuit like that of FIG. 8,where the model can be implemented using a variety of circuits. Thelocal clock recovery circuit is represented in the same basic way as inFIG. 7. Line 500 receives the data phase signal φ_(DATA) and provides itas an input to a summing node 501. The output of the summing node 501 isapplied to the phase error detector gain 502, where it is multiplied byan effective phase detector gain (K_(PD)) to generate an early/latesignal. This signal is then applied to the loop filter, which includes asecond order or integrating element 503 (K_(I)(1−z⁻¹)), and a firstorder or proportional element 504 (K_(P)). Note that a single orderfilter can instead be used if desired. The outputs of the elements 503and 504 are summed at node 505 and provided to a digital phasecontroller 506 (K_(DPC)/(1−z⁻¹)). The output of summing node 507 is thenfed back on line 508 to error detection node 501, and also provides thelocal recovered clock φ_(CDR).

The global clock recovery circuit receives the early/late signal fromthe error detector 502, and similar signals from other local clockrecovery circuits at a summing node 509. The output of this node is thenapplied to a frequency tracking circuit including elements 521-523, andin parallel, to a phase adjusting circuit including elements 525 and526. The outputs of these respective circuits are then summed at node524 to produce the global recovered clock φ_(R-CK).

The frequency tracking circuit is represented by an accumulator 521(K_(FREQ)/(1−Z⁻¹)²), a Delta-Sigma modulator 522 (Δ-Σ) and a phaselocked loop 523 (H_(PLL)). The phase adjusting circuit is represented bya phase adjustment element 525 (K_(P)) and a filter 526. Filter 526 isrepresented as a band pass filter in accordance with the delay lockedloop dynamics discussed in regard to FIG. 8. In that context, thehigh-pass cutoff frequency is typically defined by the upper range ofthe loop bandwidth of the delay locked loop; that is, a high pass cutoffis provided by the maximum frequency at which the error detector outputcan adjust the delay of delay line. Although not shown in the diagrams,the global clock recovery circuit is configurable in some embodiments,by the use of control registers for example to set the proportional andintegral gain values of components of the circuits.

Generally, the circuits 98 and 130 shown in FIGS. 4 and 6, respectively,and the circuit illustrated block 435 in FIG. 9, comprise variousembodiments of means, coupled to the plurality of receivers, forgenerating the global recovered clock in response to the local errorsignals. Such embodiments include those configurable using controlregisters. Also, the circuits shown in FIG. 8, corresponding to thefrequency tracker 301 and the phase adjuster 300, including embodimentsconfigurable using control registers, correspond to means, coupled tothe plurality of receivers, for generating the global recovered clock inresponse to the local error signals.

Note that the circuits described above provide for substantially reducedjitter. The optional use of global frequency tracking, that is, where anoscillation source is controlled responsive to averaged or accumulatedlocal clock recovery circuits, provides for significantly less ditherjitter; as mentioned, in specific embodiments, this circuit can berooted in a voltage controlled oscillator that generates an oscillationfrequency that closely tracks frequency an idealized source clock(representing correlation between the various embedded clocks). Othercircuits can also be used. The optional use of a shared delay line toprovide band-limited, low latency jitter correction permits cancelationof correlated jitter represented by the various embedded clocks (e.g.,power supply induced jitter), with low frequency correction beingaddressed by other circuitry, and high frequency correction beinglimited by the update rate of the shared delay line. In specificembodiments indicated above, this delay line can implemented in the formof voltage- or current-controlled delay elements.

FIG. 11 illustrates a jitter transfer function (from Φ_(DATA) toΦ_(CDR)) showing the benefit of using a shared delay line as described.In FIG. 11, the vertical axis represents a ratio of Φ_(DATA) to Φ_(CDR),while the horizontal axis represents jitter frequency. The jittertransfer function includes a first component 601 that corresponds to alow pass transfer function of the local clock recovery circuit with acutoff frequency indicated by point 605 on the graph. The cutofffrequency of the entire clock recovery circuit is increased beyond thiscutoff frequency by the use of global delay line to compensate for highfrequency correlated jitter, as represented by a second jitter transferfunction component 602. The combined jitter transfer functionrepresented by trace 603 on the graph in FIG. 11 provides substantiallyimproved performance for data communications rooted in multiple embeddedclocks with correlated high-frequency jitter.

FIG. 12 illustrates a method for clock recovery in a multi-lane datacommunication system between devices. The method includes receivinginput data signals at a plurality of receivers at local sampling timesresponsive to a global recovered clock (801), providing local errorsignals from the plurality of receivers, the local error signalsindicating timing differences between respective input data signals andrespective local sampling times (802), and providing a global recoveredclock with a frequency tracking the input data signals, as a function ofa combination of the local error signals (803). As indicated forembodiments above, the frequency tracking can be performed using acontrollable oscillation source that generates a frequency closelymatched to a clock source presumptively used at some point in the clockgeneration paths for the various input data signals. These techniquescan be applied to substantially minimize the phase step sizes (e.g., anddither jitter) applied in local data signal clock recovery.

FIG. 13 illustrates another method for clock recovery in a multi-lanedata communication system between devices. The method includes receivinginput data signals at a plurality of receivers at local sampling timesresponsive to a global recovered clock (901), providing local errorsignals from the plurality of receivers, the local error signalsindicating timing differences between respective input data signals andrespective local sampling times (902), generating a combined local errorsignals based on the local error signals from the plurality of receivers(903), and producing global recovered clock having a phase adjusted as aband pass function of a combination of the local error signals (904), byfor example using a delay locked loop including a voltage or currentcontrolled delay line in response to the combination of the local errorsignals and the global recovered clock. As indicated, this method can beapplied to correct for high frequency jitter correlated amongst theembedded clocks of the various input data signals.

FIG. 14 illustrates a yet another method for clock recovery in amulti-lane data communication system between devices. The methodincludes receiving input data signals at a plurality of receivers atlocal sampling times responsive to a global recovered clock (1001),providing local error signals from the plurality of receivers, the localerror signals indicating timing differences between respective inputdata signals and respective local sampling times (1002), providing afrequency tracking clock with a frequency tracking the input datasignals, as a function of a first combination of the local error signals(1003), and adjusting phase of the frequency tracking clock foralignment with the input data signals as a function of a secondcombination of the local error signals to provide the global recoveredclock (1004). Once again, in specific embodiments, this can optionallybe performed using a controllable oscillation source to perform thefrequency tracking, and using a voltage- or current-controlled delayline to adjust the global recovered clock within a specific frequencyband.

A shared global CDR combined with per-lane, local digital CDRs isdescribed. Rather than using only the early/late information of a singledata lane, combined early/late information from a set of data lanes isused for the global CDR. Early/late information from different datalanes can be combined linearly to estimate the timing error. This alsoreduces the effect of data-dependent jitter on recovered clocks. As anoptional feature, the global CDR can be based in a controllableoscillation source such that a frequency for the global clock isgenerated that closely approximates a shared source presumptively usedfor the various transmit clocks, that is, as opposed to phase ditheringback and forth to attempt match this frequency. This can be implementedfor example using a voltage controlled oscillator to generate the globalrecovered clock. As a second optional feature, a shared DLL can be usedto track out the high frequency jitter that is common to (or correlatedbetween) all data lanes. To reduce latency, a simple analog loop can beused with a voltage or current controlled delay line. Voltage or currentcontrolled delay lines can be used that have a limited deskew range,such as on the order of 1-2 UI. This is acceptable since it is possibleto reduce the mid-frequency jitter (i.e. in the range of about 30MHz-150 MHz) within 2 UI. In addition, to avoid saturating the delayline, band pass phase transfer characteristics are applied in the sharedDLL. Based on simulation results, the shared DLL bandwidth can be higherthan 100 MHz.

In a typical multi-lane SERDES application, a frequency offset betweenincoming data and receiver side reference clock PLL is common for allthe local CDRs. Hence, a frequency tracking loop can be shared asdescribed herein. The shared frequency tracking can be fundamentallydifferent than conventional approaches, because rather than using thephase interpolator in the local CDR lane to compensate for the frequencyoffset, the VCO of the shared PLL is re-aligned to the incoming datafrequency. This can be done by adjusting the division ratio in the PLLfeedback path as shown in the illustrated examples.

There are several advantages to the approaches described here. First,the quantization noise is low pass filtered by the shared PLL bandwidth,which significantly reduces the dithering jitter. This advantage can befurther enhanced by inserting a Delta-Sigma modulator in the frequencytracking path. Second, in the conventional solution, the use of digitalphase adjusters to compensate for the frequency offset causes bothintegral non-linearity (INL) and differential non-linearity (DNL) totranslate to dithering jitter. This however is not the case in someapproaches described herein. Since the local phase interpolators incircuits described herein do not need to compensate for the frequencyoffset, local CDR dithering is only limited to the DNL of the phaseinterpolator.

It should be noted that the various circuits disclosed herein may bedescribed using computer aided design tools and expressed (orrepresented), as data and/or instructions embodied in variouscomputer-readable media, in terms of their behavioral, registertransfer, logic component, transistor, layout geometries, and/or othercharacteristics. Formats of files and other objects in which suchcircuit expressions may be implemented include, but are not limited to,formats supporting behavioral languages such as C, Verilog, and VHDL,formats supporting register level description languages like RTL, andformats supporting geometry description languages such as GDSII, GDSIII,GDSIV, CIF, MEBES and any other suitable formats and languages. A memoryincluding computer-readable media in which such formatted data and/orinstructions may be embodied include, but are not limited to, computerstorage media in various forms (e.g., optical, magnetic or semiconductorstorage media, whether independently distributed in that manner, orstored “in situ” in an operating system).

When received within a computer system via one or more computer-readablemedia, such data and/or instruction-based expressions of the abovedescribed circuits may be processed by a processing entity (e.g., one ormore processors) within the computer system in conjunction withexecution of one or more other computer programs including, withoutlimitation, net-list generation programs, place and route programs andthe like, to generate a representation or image of a physicalmanifestation of such circuits. Such representation or image maythereafter be used in device fabrication, for example, by enablinggeneration of one or more masks that are used to form various componentsof the circuits in a device fabrication process.

As described above, per-lane digital CDR can be simplified to optimizesteady state sampling point and track only low frequency jitter. Ashared analog loop can be used both to track high frequency jitter andto track frequency offset, thereby achieving better jitter tolerance.Latency can be reduced by keeping the delay locked loop analog. Sincepower used in the shared loop can be amortized over multiple data lanes,actual power penalty is not significant.

A shared PLL can be used to filter the quantization noise of the localDigital CDRs, decoupling the digital clock rate at the local CDR fromfrequency offset tracking. As a result, both phase and frequencyresolution can be much higher in the clock recovery circuits.

A shared DLL is described that provides band pass, low latency jittertracking. The band-pass characteristic of the shared DLL provides goodcorrelated jitter tracking and combining data from multiple lanes withdifferent data provides less sensitivity to data dependent jitter DDJ.

Combinations of above methods achieve both frequency offset tracking andhigh frequency jitter tracking.

An example described herein can be characterized as a CDR with sharedfrequency tracking which adjusts a shared VCO based on inputs frommultiple lanes between two devices sharing a common frequency source. Inaddition, examples of the CDR described herein include quantizationnoise shaping to further reduce the dithering jitter.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, any of the specific numbers ofbits, signal path widths, signaling or operating frequencies, componentcircuits or devices and the like may be different from those describedabove in alternative embodiments. In other instances, well-knowncircuits and devices are shown in block diagram form to avoid obscuringthe present invention unnecessarily. Additionally, lanes or otherinterconnections between integrated circuit devices or internal circuitelements or blocks may be shown as buses or as single signal lines. Eachof the buses may alternatively be a single signal line, and each of thesingle signal lines may alternatively be buses. Signals and signalinglanes, however shown or described, may be single-ended or differential.A signal driving circuit is said to “output” a signal to a signalreceiving circuit when the signal driving circuit asserts (or deasserts,if explicitly stated or indicated by context) the signal on a signalline coupled between the signal driving and signal receiving circuits.“Clock” is used herein to refer to a periodic timing signal used tocoordinate actions between circuits on one or more integrated circuitdevices. The term “coupled” is used herein to express a directconnection as well as a connection through one or more interveningcircuits or structures. Integrated circuit device “programming” mayinclude, for example and without limitation, loading a control valueinto a register or other storage circuit within the device in responseto a host instruction and thus controlling an operational aspect of thedevice, establishing a device configuration or controlling anoperational aspect of the device through a one-time programmingoperation (e.g., blowing fuses within a configuration circuit duringdevice production), and/or connecting one or more selected pins or othercontact structures of the device to reference voltage lines (alsoreferred to as strapping) to establish a particular device configurationor operation aspect of the device. The terms “exemplary” and“embodiment” are used to express an example, not a preference orrequirement.

While the invention has been described with reference to specificembodiments thereof, it will be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope. For example, features or aspects of any of the embodimentsmay be applied, at least where practicable, in combination with anyother of the embodiments or in place of counterpart features or aspectsthereof. Accordingly, the specification and drawings are to be regardedin an illustrative rather than a restrictive sense.

1. (canceled)
 2. An integrated circuit to couple to conductive signalpaths to receive data signals respective to the signal paths each from asource, each of the data signals conveying an embedded clock, theintegrated circuit comprising: a global clock generator; and receiversto receive and sample respective ones of the data signals, each receiverto generate phase error information representing difference in timingbetween (a) a sampling clock used by the respective receiver to samplethe respective data signal and (b) timing information represented by theembedded clock conveyed by the respective data signal; wherein theglobal clock generator is to generate a global clock responsive to thephase error information generated by each of the receivers, and eachreceiver also has a local clock generation circuit to generate thesampling clock used by the respective receiver to sample the respectivedata signal, in dependence on the global clock and the phase errorinformation generated by the respective receiver.
 3. The integratedcircuit of claim 2, wherein the global clock generator comprises avariable frequency oscillator to generate the global clock to have afrequency corresponding to accumulated error, the accumulated errorcorresponding to aggregation of the phase error information generated byeach of the receivers.
 4. The integrated circuit of claim 3, wherein:the global clock generator further comprises a delay line to delay theglobal clock according to a combination of the phase error informationgenerated by the receivers, to responsively generate a delayed clock;the local clock generation circuit for each receiver is to generate thesampling clock used by the respective receiver in dependence on thedelayed clock; and the delay line has a loop bandwidth which is fasterthan a loop bandwidth associated with the variable frequency oscillator.5. The integrated circuit of claim 2, wherein: the global clockgenerator comprises a delay line to produce the global clock to have aphase dependent on a combination of the phase error informationgenerated by each of the receivers; and the local clock generationcircuit for each receiver comprises a local phase adjustment circuit toproduce the sampling clock used by the respective receiver in dependenceon difference between (a) the phase of the global clock produced by thedelay line and (b) the timing information represented by the embeddedclock conveyed by the respective data signal.
 6. The integrated circuitof claim 2, wherein: the global clock generator comprises a frequencytracker and a phase tracker; the phase tracker has a first loopbandwidth, to adjust phase of the global clock in dependence on thephase error information generated each of the receivers and to track anaverage phase of the timing information represented by the embeddedclocks by the respective data signals; the frequency tracker has asecond loop bandwidth, to adjust frequency of the global clockresponsive to phase error accumulated from each of the receivers whichis not corrected by the phase tracker.
 7. The integrated circuit ofclaim 2, wherein the integrated circuit is to generate the global clockin dependence on a reference clock that is generated independently ofthe timing information represented by the embedded clocks conveyed bythe data signals.
 8. The integrated circuit of claim 2, wherein theintegrated circuit is to generate the global clock in dependence on areference clock, and wherein the timing information represented by theembedded clocks conveyed by the data signals is such that the timinginformation is also generated in dependence on the reference clock. 9.The integrated circuit of claim 2, wherein the local clock generationcircuit for each receiver comprises a bang-bang phase detector togenerate the phase error information for the respective receiverdependent on difference between timing of data symbol transitions of therespective data signal and edges of the sampling clock used by therespective receiver.
 10. The integrated circuit of claim 2, whereinthere are four conductive signal paths and four corresponding datasignals, and wherein the integrated circuit comprises four of thereceivers and four local clock generation circuits.
 11. The integratedcircuit of claim 2, wherein there is one source, wherein the datasignals respective to the signal paths originate in common from the onesource, and wherein the data signals respective to the signal paths aretransmitted in dependence on shared timing.
 12. An integrated circuit tocouple to conductive signal paths to receive data signals respective tothe signal paths each from a source, each of the data signals conveyingan embedded clock, the integrated circuit comprising: a global clockgenerator; and receivers to receive and sample respective ones of thedata signals, each receiver to generate an early/late signalrepresenting phase error between (a) a sampling clock used by therespective receiver to sample the respective data signal and (b) timinginformation represented by the embedded clock conveyed by the respectivedata signal; wherein the global clock generator is to generate a globalclock responsive to the early/late signals generated by the receivers,and each receiver also has a local clock generation circuit to generatethe sampling clock used by the respective receiver to sample therespective data signal, in dependence on the global clock and theearly/late signal generated by the respective receiver.
 13. Theintegrated circuit of claim 12, wherein the global clock generatorcomprises a variable frequency oscillator to generate the global clockto have a frequency corresponding to accumulated error, the accumulatederror corresponding to aggregation of the early/late signals generatedby the receivers.
 14. The integrated circuit of claim 13, wherein: theglobal clock generator further comprises a delay line to delay theglobal clock according to a combination of the early late signalsgenerated by the receivers, to responsively generate a delayed clock;the local clock generation circuit for each receiver is to generate thesampling clock used by the respective receiver in dependence on thedelayed clock; and the delay line has a loop bandwidth which is fasterthan a loop bandwidth associated with the variable frequency oscillator.15. The integrated circuit of claim 12, wherein: the global clockgenerator comprises a delay line to produce the global clock to have aphase dependent on a combination of the early/late signals generated bythe receivers; and the local clock generation circuit for each receivercomprises a local phase adjustment circuit to produce the sampling clockused by the respective receiver in dependence on difference between (a)the phase of the global clock produced by the delay line and (b) thetiming information represented by the embedded clock conveyed by therespective data signal.
 16. The integrated circuit of claim 12, wherein:the global clock generator comprises a frequency tracker and a phasetracker; the phase tracker has a first loop bandwidth, to adjust phaseof the global clock in dependence on the early/late signal generatedeach of the receivers and to track an average phase of the timinginformation represented by the embedded clocks by the respective datasignals; the frequency tracker has a second loop bandwidth, to adjustfrequency of the global clock responsive to phase error represented byaccumulation of the early/late signals from the receivers where suchphase error is not corrected by the phase tracker.
 17. The integratedcircuit of claim 12, wherein the integrated circuit is to generate theglobal clock in dependence on a reference clock that is generatedindependently of the timing information represented by the embeddedclocks conveyed by the data signals.
 18. The integrated circuit of claim12, wherein the integrated circuit is to generate the global clock independence on a reference clock, and wherein the timing informationrepresented by the embedded clocks conveyed by the data signals is suchthat the timing information is also generated in dependence on thereference clock.
 19. The integrated circuit of claim 12, wherein thereis one source, wherein the data signals respective to the signal pathsoriginate in common from the one source, and wherein the data signalsrespective to the signal paths are transmitted in dependence on sharedtiming.
 20. An integrated circuit to couple to conductive signal pathsto receive data signals respective to the signal paths each from asource, each of the data signals conveying an embedded clock, theintegrated circuit comprising: a global clock generator to generate aglobal clock; and receivers to receive and sample respective ones of thedata signals, each receiver to generate phase error informationrepresenting difference in timing between (a) a sampling clock used bythe respective receiver to sample the respective data signal and (b)timing information represented by the embedded clock conveyed by therespective data signal; wherein the global clock generator comprises afrequency tracker and a phase tracker, the phase tracker has a firstloop bandwidth, to adjust phase of the global clock in dependence on thephase error information generated each of the receivers and to track anaverage phase of the timing information represented by the embeddedclocks by the respective data signals, the frequency tracker has asecond loop bandwidth, to adjust frequency of the global clockresponsive to phase error accumulated from each of the receivers whichis not corrected by the phase tracker, and each receiver also has alocal clock generation circuit to generate the sampling clock used bythe respective receiver to sample the respective data signal, independence on the global clock and the phase error information generatedby the respective receiver.
 21. The integrated circuit of claim 20,wherein the frequency tracker comprises a variable frequency oscillatorto generate the global clock to have a frequency corresponding toaccumulated error, the accumulated error corresponding to aggregation ofthe phase error information generated by the receivers.
 22. Theintegrated circuit of claim 20, wherein: the phase tracker comprises adelay line to produce the global clock to have a phase dependent on acombination of the phase error information generated by the receivers;and the local clock generation circuit for each receiver comprises alocal phase adjustment circuit to produce the sampling clock used by therespective receiver in dependence on difference between (a) the phase ofthe global clock produced by the delay line and (b) the timinginformation represented by the embedded clock conveyed by the respectivedata signal.
 23. The integrated circuit of claim 20, wherein there isone source, wherein the data signals respective to the signal pathsoriginate in common from the one source, and wherein the data signalsrespective to the signal paths are transmitted in dependence on sharedtiming.